Secular equilibrium occurs when the abundance of a radioactive isotope remains constant because it is being produced at the same rate that it is decaying. This occurs in nature in the two U-Pb decay series. 238U does not decay directly to 206Pb but passes through a series of 18 different intermediate isotopes before finally decaying to the stable 206Pb. The same is true for the decay of 235U to 207Pb through 13 intermediate isotopes. In a closed system, such as that of a mineral grain, each of these 31 intermediate isotopes will reach secular equilibrium given sufficient time.

In principle, secular equilibrium is simple to determine because when an isotope has reached this state the concentration of its parent isotope times its decay constant equals the concentration of the daughter isotope times its decay constant. 238U decays directly to 234Th, so when 234Th achieves secular equilibrium, [238U] x DC-238U = [234Th] x DC-234Th. The decay constants have been determined in physics laboratories and the abundances of the various isotopes can be measured using a mass spectrometer. In practice, many of the intermediate isotopes are too short-lived to have enough of an abundance to be measured and a few of the spectacularly short-lived ones will reach secular equilibrium before the rock has cooled enough for you to sample it. Both U decay series contain several isotopes that have long enough half-lives that their abundances can be measured.

In geochronology secular equilibrium serves as a robust check for closed system behavior in minerals being dated by conventional U-Pb methods. Open system behavior in the last 1.5 million years will manifest itself as secular disequilibrium. It can also be used as a dating method in its own right, making it useful for dating geological materials that are younger than 1.5 million years.

Secular equilibrium represents an insurmountable problem for Young Earth Creationists because it contains multiple independent checks of a material's age reaching back as far as 1.5 million years. Given the sensitivity and errors on most techniques, it takes a closed system about six times the half-life of the isotope of interest to get close enough to secular equilibrium that we can no longer tell the difference. More if you have good equipment and techniques. The isotope 234U has a half-life of 245,500 year, placing an easily obtainable lower age limit on any material exhibiting 234U secular equilibrium of 1.5 million years. The overwhelming majority of rocks on Earth show 234U secular equilibrium and thus must be older than 1.5 Ma.

Pre-emptive counter-arguments:

God made the rocks that way with secular equilibrium pre-established. You're painting a picture of an extremely dishonest Creator. Moreover, there's no possible way to falsify your assertion. Kindly desist or we'll feed you to the Last-Thursdayists.

The radioactive decay rates haven't always been what they are today. There's no good reason to believe this as it flies in the face of everything we understand about nuclear physics. Furthermore, you'll have to change several dozen decay rates simultaneously and all in lock-step with each other. You're now painting a picture of a sneaky and dishonest Creator. Oh, and it will have unfortunate consequences for the planet detailed below.

Universal constants were different back then allowing for a lot of radioactive decay in just a little time. Fiddling with universal constants such as the speed of light or nuclear weak force is a very bad idea that will tend to explode the universe or snuff it out like a candle. Assuming you somehow avoid that fate you are now going to release ~4.5 billion years worth of radioactivity in a few thousand years. That means you'll be increasing natural radioactivity by more than a factor of a million. The radioactivity alone would sterilize the Earth. Radioactive decay also produces heat, a fun phenomenon that is responsible for Three Mile Island, Fukushima, the molten core of the Earth and the fact that crust over ~30 km thick starts to melt under its own accumulated heat. At a million times the radioactivity, the radiogenic heat will melt the crust entirely and the Earth will shine like a star. Today, thousands of years later, the Earth would consist of a super-heated lava ocean surrounded by an atmosphere dominated by radiogenic helium with lesser amounts of various radiolytic H-N-O compounds. The garden of Eden and the rest of scripture won't survive.